Use FlashAttention for multi_query_kv_attention (#4)

Author: WoosukKwon

README.md

@@ -4,6 +4,7 @@
 
 ```bash
 pip install cmake torch transformers
+pip install flash-attn # This may take up to 10 mins.
 pip install -e .
 ```
 

cacheflow/models/attention.py

@@ -1,5 +1,6 @@
 from typing import List, Optional
 
+from flash_attn.flash_attention import FlashAttention
 import torch
 import torch.nn as nn
 
@@ -14,20 +15,7 @@ def __init__(self, scale: float) -> None:
         super().__init__()
         self.scale = float(scale)
 
-    def _masked_attention(
-        self,
-        query: torch.Tensor,                        # [num_queries, num_heads, head_size]
-        key: torch.Tensor,                          # [num_keys, num_heads, head_size]
-        value: torch.Tensor,                        # [num_keys, num_heads, head_size]
-        attn_mask: Optional[torch.Tensor] = None,   # [num_queries, num_keys]
-    ) -> torch.Tensor:                              # [num_queries, num_heads, head_size]
-        query = query * self.scale
-        attn = torch.einsum('qhd,khd->hqk', query, key)
-        if attn_mask is not None:
-            attn = attn + attn_mask
-        attn = torch.softmax(attn, dim=-1)
-        out = torch.einsum('hqk,khd->qhd', attn, value)
-        return out
+        self.flash_attn = FlashAttention(softmax_scale=self.scale)
 
     def multi_query_kv_attention(
         self,
@@ -37,21 +25,31 @@ def multi_query_kv_attention(
         value: torch.Tensor,        # [num_prompt_tokens, num_heads, head_size]
         prompt_lens: List[int],
     ) -> None:
-        # FIXME(woosuk): Replace the following with a custom op.
-        start_idx = 0
+        if query.dtype == torch.float:
+            raise ValueError('The float data type is not supported by '
+                             'FlashAttention. Use the half data type instead.')
+        head_size = query.shape[2]
+        if head_size > 128:
+            raise ValueError('FlashAttention does not support head_size > 128.')
+
+        device = query.device
+        prefix_sum = [0]
         for prompt_len in prompt_lens:
-            out = output[start_idx:start_idx + prompt_len]
-            q = query[start_idx:start_idx + prompt_len]
-            k = key[start_idx:start_idx + prompt_len]
-            v = value[start_idx:start_idx + prompt_len]
-
-            attention_mask = torch.triu(
-                torch.ones(q.shape[0], k.shape[0]), diagonal=1) * -1e5
-            attention_mask = attention_mask.to(dtype=q.dtype, device=q.device)
-            attention_out = self._masked_attention(q, k, v, attention_mask)
-            out.copy_(attention_out, non_blocking=True)
-
-            start_idx += prompt_len
+            prefix_sum.append(prefix_sum[-1] + prompt_len)
+        prefix_sum = torch.tensor(prefix_sum, dtype=torch.int, device=device)
+        max_prompt_len = max(prompt_lens)
+
+        # FIXME(woosuk): Unnecessary copy. Optimize this.
+        qkv = torch.stack([query, key, value], dim=1)
+        out = self.flash_attn(
+            qkv,
+            cu_seqlens=prefix_sum,
+            max_s=max_prompt_len,
+            causal=True,
+        )[0]
+        num_tokens = prefix_sum[-1]
+        # FIXME(woosuk): Unnecessary copy. Optimize this.
+        output[:num_tokens].copy_(out, non_blocking=True)
 
     def single_query_cached_kv_attention(
         self,
@@ -61,6 +59,14 @@ def single_query_cached_kv_attention(
         value_cache: torch.Tensor,      # [num_blocks, num_heads, head_size, block_size]
         input_metadata: InputMetadata,
     ) -> None:
+        head_size = value_cache.shape[2]
+        supported_head_sizes = [32, 64, 80, 96, 128, 160, 192, 256]
+        if head_size not in supported_head_sizes:
+            raise ValueError(f'head_size ({head_size}) is not supported by '
+                             'the single_query_cached_kv_attention kernel. '
+                             'Use one of the following head sizes: '
+                             f'{supported_head_sizes}.')
+
         block_size = value_cache.shape[3]
         attention_ops.single_query_cached_kv_attention(
             output,
@@ -101,8 +107,9 @@ def forward(
         output = output.view(-1, num_heads, head_size)
 
         # Compute the attention op for prompts.
-        self.multi_query_kv_attention(
-            output, query, key, value, input_metadata.prompt_lens)
+        if input_metadata.num_prompts > 0:
+            self.multi_query_kv_attention(
+                output, query, key, value, input_metadata.prompt_lens)
 
         # Wait until the cache op is done.
         if cache_event is not None:

server.py

@@ -9,10 +9,12 @@
 parser.add_argument('--model', type=str, default='facebook/opt-125m', help='model name')
 parser.add_argument('--num-nodes', type=int, default=1, help='number of nodes')
 parser.add_argument('--num-workers', type=int, default=1, help='number of workers per node')
-parser.add_argument('--block-size', type=int, default=8, help='token block size')
+parser.add_argument('--block-size', type=int, default=8, choices=[8, 16], help='token block size')
 # TODO(woosuk): Add an analytical model to determine the maximum number of GPU/CPU blocks.
 parser.add_argument('--num-gpu-blocks', type=int, default=1024, help='number of GPU blocks (per GPU)')
-parser.add_argument('--num-cpu-blocks', type=int, default=256, help='number of CPU blocks (per GPU)')
+parser.add_argument('--num-cpu-blocks', type=int, default=32, help='number of CPU blocks (per GPU)')
+# NOTE(woosuk): If FlashAttention is used, the float data type is not supported.
+parser.add_argument('--dtype', type=str, default='half', choices=['half', 'float'], help='data type')
 args = parser.parse_args()
 
 
@@ -27,6 +29,7 @@ def main():
             block_size=args.block_size,
             num_gpu_blocks=args.num_gpu_blocks,
             num_cpu_blocks=args.num_cpu_blocks,
+            dtype=args.dtype,
         )
         controllers.append(controller)
 

tests/kernels/attention.py

@@ -1,10 +1,13 @@
 import random
 from typing import Optional
 
+from flash_attn.flash_attention import FlashAttention
 import torch
 
 from cacheflow import attention_ops
 
+MAX_SEQ_LEN = 4096
+
 
 def ref_masked_attention(
     query: torch.Tensor,
@@ -79,7 +82,7 @@ def test_single_query_cached_kv_attention(
     value_cache = torch.randn(
         size=(num_blocks, *value_block_shape), dtype=dtype, device='cuda')
 
-    context_lens = [random.randint(1, 4096) for _ in range(num_tokens)] 
+    context_lens = [random.randint(1, MAX_SEQ_LEN) for _ in range(num_tokens)] 
     max_context_len = max(context_lens)
     context_lens = torch.tensor(context_lens, dtype=torch.int, device='cuda')
 
@@ -123,11 +126,60 @@ def test_single_query_cached_kv_attention(
     assert torch.allclose(output, ref_output, atol=1e-3, rtol=1e-5)
 
 
+def test_multi_query_kv_attention(
+    num_seqs: int,
+    num_heads: int,
+    head_size: int,
+    dtype: torch.dtype,
+) -> None:
+    seq_lens = random.sample(range(1, MAX_SEQ_LEN), num_seqs)
+    max_seq_len = max(seq_lens)
+    num_tokens = sum(seq_lens)
+
+    cu_seq_lens = [0]
+    for seq_len in seq_lens:
+        cu_seq_lens.append(cu_seq_lens[-1] + seq_len)
+    cu_seq_lens = torch.tensor(cu_seq_lens, dtype=torch.int, device='cuda')
+
+    scale = float(1.0 / (head_size ** 0.5))
+    query = torch.randn(
+        num_tokens, num_heads, head_size, dtype=dtype, device='cuda')
+    key = torch.rand_like(query)
+    value = torch.rand_like(query)
+
+    qkv = torch.stack([query, key, value], dim=1)
+    flash_attn = FlashAttention(softmax_scale=scale)
+    output = flash_attn(
+        qkv,
+        cu_seqlens=cu_seq_lens,
+        max_s=max_seq_len,
+        causal=True,
+    )[0]
+
+    ref_outputs = []
+    for i, seq_len in enumerate(seq_lens):
+        attn_mask = torch.triu(torch.ones(seq_len, seq_len), diagonal=1) * -1e5
+        attn_mask = attn_mask.to(dtype=dtype, device='cuda')
+        start_idx = cu_seq_lens[i]
+        end_idx = cu_seq_lens[i + 1]
+        ref_output = ref_masked_attention(
+            query[start_idx:end_idx],
+            key[start_idx:end_idx],
+            value[start_idx:end_idx],
+            scale,
+            attn_mask=attn_mask,
+        )
+        ref_outputs.append(ref_output)
+    ref_output = torch.cat(ref_outputs, dim=0)
+
+    assert torch.allclose(output, ref_output, atol=1e-3, rtol=1e-5)
+
+
 @torch.inference_mode()
 def test_attention() -> None:
     for dtype in [torch.half, torch.float]:
         for block_size in [8, 16]:
-            for head_size in [64, 80, 96, 128, 256]:
+            for head_size in [32, 64, 80, 96, 128, 160, 192, 256]:
                 test_single_query_cached_kv_attention(
                     num_tokens=37,
                     num_heads=3,
@@ -137,6 +189,17 @@ def test_attention() -> None:
                     dtype=dtype,
                 )
 
+    # NOTE(woosuk): FlashAttention does not support FP32.
+    for dtype in [torch.half]:
+        # NOTE(woosuk): FlashAttention does not support head_size > 128.
+        for head_size in [64, 80, 96, 128]:
+            test_multi_query_kv_attention(
+                num_seqs=11,
+                num_heads=3,
+                head_size=head_size,
+                dtype=dtype,
+            )
+
 
 if __name__ == '__main__':
     test_attention()